Polyprotic Acid Titrations
- Polyprotic acids donate more than one proton, producing multiple equivalence
points
- H₃PO₄ has 3 dissociation steps: Ka₁ >> Ka₂ >> Ka₃ (each ~10⁵× weaker)
- Each step has its own equivalence point and buffer region
- At the half-equivalence point, pH = pKₐ for that step
- The pH curve shows distinct steps — like a staircase
Reading the Staircase
- Buffer region: flat zones where pH resists change (weak acid + conjugate base)
- Half-equivalence: midpoint of each buffer region → pH = pKₐ
- Equivalence point: steep rise at each step → all protons neutralized for that
stage
- H₃PO₄: 3 equiv. points at volumes V, 2V, 3V (if NaOH:acid = 1:1 concentration)
- Third equivalence of H₃PO₄ is often too gradual to detect (Ka₃ = 4.2 × 10⁻¹³)
IB HL Exam Tips
- Key skill: identify number of exchangeable protons from the pH curve shape
- Calculating Ka: at half-equivalence, [HA] = [A⁻], so Ka = [H⁺] and pKₐ = pH
- Volume ratios: equiv points are equally spaced if same mole ratio (V, 2V, 3V…)
- Common mistake: confusing the number of equiv. points with total protons — some
steps may be too close or weak to observe